Stock vacuum petcocks available on dual sport models (DR350S and DR350SE) are known to degrade and fail after several years. Vacuum petcocks have three positions: ON, RES, and PRI. The positions are described below:
- ON - fuel flows only when the engine is running determined by vacuum present at the carburetor
- RES - reserve fuel similar to ON
- PRI - fuel flows regardless of engine state, used to initially fill (prime) the carburetor bowl if the bike has been sitting for several days
Vacuum petcocks contain a thin diaphragm that allows fuel to flow in the ON position when the engine is running. After several years, the diaphragm degrades allowing gas to leak past the diaphragm, flow down the vacuum line, and enter the carburetor. The excess fuel causes an abnormally rich air fuel mixture. An excessively rich mixture will result in poor fuel economy, poor idle, weak performance, bogging, or the engine may not run at all. It can also allow large amounts of fuel to leak into the crank case over long periods of time. Gas in the crankcase dilutes the engine oil which can lead to serious engine damage.
A common solution is to replace the vacuum petcock with a simple ON/OFF petcock from a dirt only model (DR350) or a Yamaha Raptor 660 model number 5LP-24500-01-00 ON/OFF petcock. The Raptor petcock (labeled fuelcock on Raptor parts fiche) appears to be used from model years 2001 through 2005.
If an ON-OFF petcock is used, it is critical to seal off the unused vacuum port on a CV carburetor. The engine will not run with the vacuum port open. Auto parts stores sell inexpensive vacuum caps for this purpose although any solution that seals the port completely will work.
One final option is to replace a faulty stock petcock with a vacuum-style petcock which has a true "off" position instead of prime. Several Kawasaki motorcycles have this type of petcock, including the Ninja 500 and the KLR650, which mount in the stock DR350 tank. The tank's cosmetic plastic lower section needs a small cutaway for these to fit. All components for these Kawasaki petcocks are available individually if repairs are ever needed.